Solar Sails Fly From Science Fiction Into Reality

Propelled through space using sunlight as fuel, solar sails have been a dream of space buffs for decades. Thanks to advances in materials science and electronics, two paper-thin craft have now left Earth, and a third will be on its way by the end of the year.

Solar Sails Fly From Science Fiction Into Reality

Solar Sails Fly From Science Fiction Into Reality

The invention of the sail revolutionized the way humans explored the earth. Later, it let us roam the skies. In late January, NASA's Nanosail-D unfurled its wings 400 miles above the planet's surface, bringing us one step closer to utilizing sails for deep-space exploration.

It's not the first solar sail to reach space: Japan's IKAROS mission achieved that record last May. Nor will it be the last one this year, if the Planetary Society launches LightSail-1 in November as planned. "We're coming to a point where the materials, launch capabilities and our prior knowledge will allow us to develop good solar sails and missions that can use them," says Dean Alhorn, the lead scientist for Nanosail-D.

The dream of propulsion by sunlight has been a long time in coming: Scientists have envisioned building solar-powered spacecraft since the 1920s. It's an appealing concept—freed from the constraints of a fuel tank, solar sails could potentially travel vast distances at high speeds—but actual implementation turned out to be too difficult until recently.

Unlike a solar cell, which absorbs photons and turns them into electricity, a solar sail reflects light particles off its surface. The sail material—typically Mylar or Colorless Polyamide 1—is just a fraction of a hair's width and coated with a 50-nanometer-thick layer of aluminum, reflecting almost 100 percent of the light that hits it. As photons rebound from the sail, they emit energy that nudges the sail forward.

In order to reflect as much light as possible, increasing thrust, solar sails must have ample surface area. Even 10 years ago, sails would have needed to be hundreds of meters wide, impossibly big, in order to carry camera and radio equipment. But adding surface area can also slow the sail by adding weight. As electronics have become smaller and more lightweight, engineers have been able to reduce the size of sails designed to carry them. Rip-stop fabric enabled engineers to overcome a further challenge: Deploying a huge, flimsy sail, packed and launched into space on a rocket, without tangling or tearing it.

Solar sails will eventually be able to travel up to 100,000 miles per hour, Alhorn says—about three times faster than today's spacecraft. But the next advances in solar sail technology will be in steering capabilities, says Bill Nye, director of the Planetary Society. "[NASA] got it off the ground, and that's good, but it's not controlled flight," he says. "What gave the Wright Brothers the great success they had was that they could control their planes. We're hoping to accomplish similar success with LightSail-1."